Electrical resistance elements



S pt. 13, 1955 D. B. PECK 2,717,946

ELECTRICAL RESISTANCE ELEMENTS Filed Oct. 14, 1950 1 N VEN TOR.

DAV/0 B. PfC/f United States Patent Ofiice ELECTRICAL RESISTANCEELEMENTS David B. Peck, Williiamstown, Mass, assignor to SpragueElectric Company, North Adams, Mass., a corporation of MassachusettsApplication October 14, 1950, Serial No. 190,10i9

6 Claims. (Ci. 2173) This invention relates to improved electricalelements and more particularly refers to novel electrical resistors.

Precision resistors have in the past been made principally by twoprocesses. First, metal and metal alloy wire wound resistors have beenemployed, particularly in the low resistance, high power values. ouscarbon has been deposited upon porcelain, steatite and similar ceramicbase elements. The former are characterized by good stability and by lowtemperature and voltage coefficients of resistance. However, they arerelatively expensive, particularly when high resistance values, sayabove 1 megohm, are required, because of the length and size of wirerequired. Further, they are large and bulky for such high resistancevalues, particularly in view of the low wattage normally encountered inthe high resistance applications.

Vitreous carbon resistors are cheaper and can be made in fairly closeresistance tolerances by spiral cutting of the deposit, etc. However,the temperature and voltage coefficients of resistance are inferior,particularly in resistance values in excess of 10 ohms.

Numerous other processes have been proposed to overcome the abovedisadvantages and produce a stable, inexpensive, high resistance valueunit. Metallized glass, carbon suspensions in gelatin or resin andsimilar modifications have been investigated. However, none of theseappear to meet the stringent specifications for high stability, highresistance (10 lO ohms) service.

It is an object of the present invention to overcome the foregoing andrelated disadvantages. A further object is to produce new and improvedresistance elements. A still further object is to produce insulatedresistance elements of exceptional electrical stability and physicaldurability. Additional objects will become apparent from the followingdescription and claims.

In this description:

Fig. 1 is a fragmentary sectional view of one embodiment of the. presentinvention;

Figs. 2A and 2B are partly broken away views showing other embodimentsof the present invention;

Figs. 3, 4 and 5A are fragmentary views similar to Fig. 1 of stillfurther embodiments of the present invention;

Fig. 5B shows a modified form of the embodiment of Fig. 5A;

Fig. 6A is a sectional view of yet another embodiment of the presentinvention;

Fig. 6B is a schematic diagram showing an equivalent electrical circuitprovided by the construction of Fig. 6A;

Fig. 7A is an isometric view of a construction element in accordancewith the present invention; and

Fig. 7B is a sectional view similar to Fig. 6A of a further constructionembodying the present invention made from elements of the type shown inFig. 7A.

The invention is generally concerned with a process for producingelectrical resistance elements which cornprises subjecting solidsolution of inorganic oxides containing from about 3% to about 82% ofoxides of metals selected from the class containing the metals ofSecond, vitrethe B sub-groups of groups I, IV and V in the fourth andsixth periods of the periodic table to a reducing atmosphere at atemperature between about 75 and about 600 C. This process is made thesubject of a number of improvements which lead to production of new anduseful resistance elements.

The reducibility of glasses containing lead oxide, bismuth oxide andantimony oxide and silver oxide, for example has been known for manyyears; iikewise, the surface conductivity of the glasses made up withone or more of the above oxides and subsequently reduced has been noted.Unfortunately, however, the characteristics of the known reduced glass,as a resistance element, are not entirely satisfactory, and inferior forhi-megohm applications. The present invention substantially overcomesthe difficulties of the prior art. For example, one defect has been themarked negative temperature coefficient of resistance of the resistors.Another has been the instability of the resistor as a function of time;resistors show an appreciable and progressive increase in resistancevalue with time.

According to one embodiment of the invention, the glass composition isfinely ground prior to reduction, to produce small particles, thesurfaces of which are then reduced by heating in a reducing atmosphere.The resulting powder may be incorporated in a glass melt, to produce asolid massive resistance element not heretofore attainable. The powdermay optionally be suspended in a resinous or other insulating binder toproduce a resistance ink or lacquer having characteristics superior tocarbon and graphite inks, particularly for high resistance values wherecarbon and graphite resistors are unsatisfactory.

This embodiment is illustrated in Figure 1 which shows a number ofglassparticles 10, the surfaces of which are reduced to provide aconducting colloidal suspension 11 of metal particles within the matrixand on the surface of glass 10. The reduction is accomplished on loosepowder and subsequently the particles may be joined together with abinder such as indicated at 12 as a resin, by incorporation in a glassmelt or actually by fusion of adjacent particles together by heating,preferably in a reducing atmosphere, above the softening point of theglass.

According to another embodiment of my invention, the resistant elementmay be produced and provided with a hermetic housing by a simple andinexpensive process. A tubular element of the reducible glass is treatedby passing a reducing gas through the bore of the tube under suitabletemperature conditions, reducing the inside surface only of the glasstube. Thereafter, terminal wires are inserted in short lengths of thetubing and sealed thereto, usually with an inert gas within thefreeboard.

Figures 2A and 2B illustrate this embodiment. Tube 20 fabricated from aglass composition suitable for reduction is placed in a furnace andreducing gas passed through the center of the tube. The inside surfaceof tube 20 thus is provided with a resistance coating 21. Alternately,the tube may be treated with very hot reducing gas. Figure 2B shows thetube of Figure 2A processed to produce a finished resistor. The 'tube 20is fused at the ends 22 and 24 to terminal wires 23 and 25 respectively.The free board within the sealed housing is preferably filled with aninert gas such as nitrogen. Resistance layer 21 contacts terminal wires23 and 25 at the point of fusion.

According to another embodiment of my invention, I produce a resistanceelement with unusually low temperature coefficient of resistance byproviding an inorganic insulating base with a linear temperaturecoefficient of expansion of less than about 1.O 10- C. On this 3 basethere is fused a layer of reducible glass possessing a thickness of lessthan about 0.01", whereby the expansion of the reducible glass layer isdetermined by the underlying base. Subsequently, the surface of theglass overlay may be reduced as elsewhere described.

In Figure 3 is shown an inorganic insulating base possessing a lineartemperature coefficient of expansion of less than 1.0 X 10 C. On thesurface of this is fused a layer of reducible glass 31 in a thicknessless than .01" and preferably less than about .002. The surface of thisoverlayer is reduced by reduction to form a conducting film 32. Thetemperature coefficient of resistance of the resistor thus produced willbe extremely low. Suitable materials include special silica glass,quartz and steatites. Some of the latter are particularly desirable,possessing very, very low temperature coeflicients.

According to yet another embodiment of my invention, I improve thestability of the reduced glass resistors by an oxidation of the surfaceof the glass, following the reduction treatment. As a result the surfacelayer is not appreciably sensitive to air and oxidizing atmospheres overthe temperature range normally met in resistor operation.

This is shown in Figure 4 in which a reducible glass mass is treated atappropriate temperatures in a reducing atmosphere to produce a surfacelayer of colloidal metal particles 41. After this process, the mass issubjected to heat treatment for a limited time in an oxygen atmosphereor other oxidizing atmosphere to produce an insulating surface layer 42above the conducting layer 41. Ordinarily, this process of reoxidationis conducted for about 10 minutes at a temperature corresponding to thereduction temperature.

In accordance with another embodiment of my invention, I control theresistance values obtainable from any given glass composition, reducedunder a particular set of conditions, by heating the glass member to itssoftening point and stretching the glass until the resistance hasincreased to the desired value per square or other measuring unit. Thisis applicable with plates, tubes, rods, etc.

Figure 5A shows a glass rod 50, the surface of which has been reduced toprovide resistance layer 51, as elsewhere described. Figure 5B shows therod after it has been heated preferably in a reducing gas flame aboveits softening point and drawn to a smaller diameter and accordingly ahigher resistance value per square or per unit length. 53 is the glasselement with reduced diameter containing thin resistance surface layer53. It is to be understood that the converse process may be applied, e.g., the mass may be heated above its softening point and then compressedto increase the concentration of metal particles in any given surfacelayer.

Another embodiment of my invention is concerned with the production ofresistance layers within glass laminates, for use as circuit elements infilters, networks and other multiple assemblies, or for use as voltagestress equalizing floating" foils in high voltage and high frequencycapacitors, to reduce or eliminate corona effects and increase breakdownvoltage.

Figure 6A shows a simplified cross section of a fused glass filtercircuit in which solid metal electrodes 61 and 62 are embedded in glassmass 60. Conducting layer 63 serves as a capacitor electrode as well asa partially distributed resistance element. Such an element would beconstructed by stacking or rolling of several films of reducible glass,one of which has been reduced to provide conducting layer 63. After thestacking or rolling, the assembly is heated above the melting point ofthe glass to fuse the assembly together. Figure 6B shows a schematicrepresentation of the circuit produced. Capacitor elements 61 and 62face against and are distributed along resistance element 63.

Figures 7A and 7B show a further and extremely advantageous constructionof the invention. The capacitor is wound or stacked with at least oneresistance layer on a thin glass plate which is fused into the capacitorassembly. Figure 7A shows a plate 68, a portion of one surface of whichis exposed to a reducing atmosphere at high temperatures to produceconducting layer 73. One or more of such layers is stacked withunreduced layers and capacitor electrodes to produce after fusion astructure such as that shown in Figure 78. For example, electrodes 71and 72 are separated by a glass mass in which are fused staggeredfloating electrodes 73, 74, and 75. These electrodes serve to increasethe corona starting voltage and raise the breakdown voltage of thecapacitor without appreciable effect upon the series resistance of thecapacitor. Because layers 73, 74 and 75 are extremely thin, their effectis merely to distribute the energy field uniformly between terminalelectrodes. It is to be understood that portions of the capacitorassembly may be made up with glass sheets which do not reduce under theconditions of treatment but which do fuse in the temperature range ofthe reducible glass constituents. It is well known, however, that leadsilicate glasses are characterized by unusually desirable dielectricproperties if reduction be avoided.

The reducible elements referred to herein are ordinarily selected fromthe class initially containing from about 3% to about 82% of lead oxide,silver oxide, gold oxide, antimony oxide, bismuth oxide or tin oxide.Preferred oxides are bismuth oxide and lead oxide. For a number ofreasons, lead oxide is a particularly desirable oxide for use inaccordance with the present invention. Mixtures of these oxides are alsouseful.

Other constituents of the solid solutions ordinarily consist of sodiumand/or potassium oxides, slica, calcium oxide, lithium oxide, berylliumoxide, magnesium oxide, aluminum oxide, iron oxide, zinc oxide, etc.These materials and the oxides previously specified may occur incombined form with silica or boron oxide, e. g. sodium silicate, leadsilicate, lead borate, etc. Further, fluorides and other materials maybe employed as fluxing and modifying agents. It is preferable to avoidthe presence of aluminum oxide and appreciable quantities of boronoxide.

The expression solid solution" is intended to mean the hard, solidstate, in which some non-crystalline phases are present. However, itneed not be a transparent glass and, in some cases, will be atranslucent or opaque vitreous material, as for example, an enamel.Potassium lead silicate enamels are representative of the latter.

The reduction treatment described herein appears to reduce certainoxides to the inactive metal state, in what appears to be colloidalparticles suspended in the glass.

The amount of reduction is controlled by a number of factors, e. g.temperature, material composition, time, gas pressure, boiling point ofthe reduced metal, gas diffusion rates, etc.

The reducing atmosphere may consist of hydrogen, methane, illuminatinggas, etc. While the pressure employed is normally atmospheric, it may bereduced or increased if so desired. The time is usually between about 5minutes and about 24 hours, depending upon the resistance value desired,the temperature and other factors.

According to my preferred practice, the temperature of reduction is suchthat the vapor pressure of the metal reduced does not exceed 0.0001 mm.Hg. The temperature of initial oxide reduction is an important variablein the process. The usual range of temperature is from about 75 C. toabout 600 C. while the preferred range for most materials is from aboutC. to about 400 C. The solid solutions containing lead oxide may betreated with excellent results at a temperature of from 275 C. to 375 C.With glass bodies high in silver oxide content, the temperature isgenerally from about 100 C. to about 200 C., while with glass bodiescontaining substantial amounts of bismuth oxide, the reductiontemperature is usually from about C. to about 250 C.

It appears that the reaction normally begins at the surface of the bodyexposed to the reducing gas and gradually penetrates into the mass ofthe material, the penetration depth being dependent, among otherfactors, on the time and temperature of reduction. The resistance valueusually decreases with time since more colloidal metal particles areproduced.

The concentration of reducible metal oxides depends upon the resistancevalue desired for a given shape, the molecular weight of the oxide, etc.Lead oxide, having a high molecular weight and density, may be presentin weight concentrations up to about 82%, as for example in a leadsilicate flint glass. Bismuth oxide is usually present in concentrationsfrom about 5% to about 55%; antimony oxide is usually present inconcentrations from about 5% to about 25%; and silver oxide is usuallypresent in concentrations from 3% to about 20%. When mixtures of theseoxides are present, one or more of them may be present in individualconcentrations of less than 3%, so long as the total of such oxides isat least 3%.

As indicated above the temperature reduction should be such that thevapor pressure of the reduced metal should be less than 0.0001 mm. Thiscondition obtains when the pressure of the reducing gas is approximatelyatmospheric. Higher reducing gas pressures may be employed with acorresponding increase in the temperature of reduction.

Innumerable glass compositions are applicable in accordance with theinvention. Some of these are listed in the table which follows, alongwith the resistance value per square under stated conditions ofreduction.

Reduction R L esis'ance COHlpOSll'JOD 1n h Glass percent by weight Time,Temp, g gig hrs. C.

62 P130 A a. 33 0 $110... 3 285 3X10 5% Na; .a. 52% S102". B 4 315 6x10587 PbO 11* 8% 8 325 4X10 34% SiOz *Composition H was a vitreous enamelglass applied to a steatite core.

The above examples are intended to illustrate the range of compositionswhich may be employed in accordance with the various embodiments of theinvention. The particular values desired in a finished resistanceelement can be met by selection of a suitable composition andapplication of it to the particular structure desired. For example,resistors having a value of 10 ohms have been produced by reducing adiameter rod of Composition A for 5 hours at 340 C., and cutting the rodinto an effective length of This element was then heated in a reducingatmosphere until its softening point was reached; at this point, the rodwas stretched to a length of 1 inch. Upon cooling, the resistance valuewas 1.8 X 10 ohms.

As many different embodiments of this invention may be made withoutdeparting from the spirit and scope hereof, it is to be understood thatthe invention is not limited to the specific embodiments hereof exceptas defined in the appended claims.

What I claim is:

1. An insulated electrical resistance element comprising a sealed tubeof glass composed essentially of silica and other metal oxides andcontaining upon its inner surface from about 3% to about 82% of an oxideselected from the class containing silver oxide, lead oxide, gold oxide,antimony oxide, bismuth oxide and tin oxide, the inner surface of saidtube containing colloidal particles of the metals of said oxides, andterminal means for passing an electric current through said metalsurface.

2. An insulated electrical resistance element comprising a resistanceportion in the form of an elongated self supporting glass membercomposed essentially of silica and other metal oxides and containingfrom about 3% to about 82% of an oxide selected from the classcontaining silver oxide, lead oxide, gold oxide, antimony oxide, bismuthoxide, and tin oxide, only a surface portion of said glass member beingchemically reduced to an electrically conductive condition to providecolloidal particles of the metals of said oxides and terminal means forpassing an electric current through said metal surface.

3. An insulated electrical resistance element comprising a glasscomposed essentially of silica and other metal oxides and containingfrom about 61% to about lead oxide, a surface layer of which containscolloidal particles of lead, and the exposed surface of said surfacelayer being non-conductive and terminal means for passing an electriccurrent through said reduced layer.

4. A process for producing high resistance value elements whichcomprises heating in a reducing atmosphere from a temperature of about75 C. to about 600 C. a solid solution of inorganic metallic oxidescontaining from about 3% to about 82% of oxides of metals selected fromthe class consisting of silver oxide, lead oxide, gold oxide, antimonyoxide, bismuth oxide and tin oxide, the temperature of reduction beingselected so that the vapor pressure of the metals of said oxides doesnot exceed 0.0001 mm. of mercury, then reoxidizing the exposed surfaceof said surface layer.

5. An electric circuit component comprising a body of glass havingembedded electrically conductive strata generally parallel to eachother, at least one of the strata being a chemically reduced glasscomposed essentially of silica and other metal oxides, and containingfrom about 3% to about 82% of an oxide selected from the class of silveroxide, lead oxide, gold oxide, antimony oxide, bismuth oxide and tinoxide, said reduced glass being electrically conductive.

6. The invention of claim 5 in which there are two terminal electrodestrata and at least one chemically reduced stratum intermediate theterminal electrode strata.

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